The invention relates to a method and system for vacuum-evaporative film deposition on a substrate.
As used herein, "local evaporation power" means the power locally applied for producing a vapor stream for vacuum-evaporative film deposition defined as the amount of power per unit area and unit time. In other words, it is the power density of the usually-electric power input. The "evaporation rate" means the amount of vapor released per unit time. It is generally proportional to the local evaporation power, allowance being made for losses due to heat radiation and heat conduction, particularly from cooling water in a water-cooled vaporizer crucible. The "vapor source" means, very broadly, a localized zone, generally a surface portion, of a vaporizer from which the local evaporation power produces a spatially-limited vapor stream.
Examples of elongated arrays of individually-controllable vapor sources to which the invention is applicable follow.
German patent No. 24 02 111 discloses an elongated, in-line array of vapor sources. It is a row of parallel vaporizer boats directly transversed by heating current and individually controllable with respect to power. Each vaporizer boat is a separate vapor source, and the local evaporation power is proportional to the heating power of each individual vaporizer boat. It is, of course, possible to arrange similarly a plurality of water-cooled vaporizer crucibles which are heated by respectively-associated electron guns.
Published German patent application DOS No. 28 12 285 and corresponding U.S. Pat. No. 4,230,739 disclose a plurality of vapor sources from the surface of the content of a single vaporizer crucible by directing an electron beam in specific patterns onto the surface of the crucible content. One surface pattern consists of individual fields arranged in a row. The area of each field and the dwell time of the electron beam on each field produce different local evaporation powers. From each of these fields or vapor sources, there emanates a spatially-defined vapor stream which condenses as a film on a substrate disposed or moving past the vapor sources at a higher level. Similar film deposition is obtained with the thermal vaporizers mentioned above.
As a rule, the vapor streams are nonhomogeneous, at least considering the individual vapor streams collectively, producing a corresponding film-condensation pattern on the substrate. When a substrate is fixedly disposed relative to the vapor sources, this results in spots with a deposited-film thickness different from that on the rest of the substrate. When the substrate normally traverses the axis of the elongated vaporizer array, a markedly nonhomogeneous collective vapor stream may even produce a so-called "stripe pattern" which is totally unsuited for many applications. For example, in coating a continuous strip or foil wound at a right angle to an in-line array of vapor sources with an electrically-conducting film, locally-varying surface resistivities result or, in coating large-size glass sheets for glazing buildings with an optically-active film, optically objectionable transmittance or reflectance characteristics result.
The nonhomogeneity of the vapor streams is due not only to unintentional differences in the adjustment of the local evaporation powers, but also to variations attributable to geometric factors. For example, thermal vaporizers at the end of an in-line array are subject to heat losses in three directions; whereas, in the center, heat losses can occur only in two directions. Also, as a result of the overlapping of adjacent vapor streams, known as vapor lobes, toward the center of an in-line array, the rates of deposition there are higher than from elsewhere along the array. This effect might be compensated for by using an in-line array of a length considerably greater than the width of the substrate to be vapor coated, but this would entail corresponding losses of vapor material as well as fouling of the system through vapor deposition on its wall surfaces instead of the substrate to be coated.
The above relationships and steps to be taken to compensate for their effects in coating a continuous-strip substrate are described very graphically in U.S. Pat. No. 3,432,355. The film-thickness distribution perpendicular to the direction of deposition motion of a material substrate has, therefore, not been left to chance.
For example, German design patent No. 1,978,459 further discloses continuously monitoring the film-thickness distribution visually by placing a fluorescent lamp downstream of vapor deposition of an optical film on a moving foil. When the film thickness varies, the local evaporation power of an in-line vaporizer can be appropriately adjusted where the transmittance is too high or too low until a uniform film-thickness distribution is obtained, at least so far as can be determined visually.
This approach, however, is only suitable with optically-transmitting films and not with optically-opaque films such as, for example, metal coatings of appreciable thickness. Besides, the measurement and control attainable are not accurate enough to satisfy present-day film-thickness uniformity requirements.